Analysis of wafer bonding by infrared transmission

In solid state technology, two silicon wafers can be combined by wafer bonding, Small particles between the two surfaces may cause voids. For quality control: the voids can be detected by transmission of infrared light: provided by a Nd:YAG laser at a wavelength of 1.064 mu m, where silicon is transparent. The defects are made visible via a CCD-camera on the monitor of a computer and appear as interference rings. This paper describes a method to enhance the visibility of the defect relative to other superimposed patterns e.g. from metallization. The voids are deformed elastically by applicating an external air pressure inside an inspection chamber. Insufficient bonded wafers; caused by particles as small as 1/2 mu m, can be detected by routine in order to enhance the yield.